When reading or writing data to the data side of a CD, conventional use of a FES (focus error signal) provides information that allows operation of a closed-loop feedback circuit to keep the optical pickup unit (OPU) focused on the data pits defined on an upper surface of a plastic layer.
However, emerging technology makes it possible to write to the label side of the CD, thereby producing an image, text and/or graphics. Unfortunately, conventional use of a FES to focus on the label side of the disk is ineffective.
An initial difficulty in focusing on the label side of the disk is that the FES signal provides a low signal-to-noise ratio, in part due to the nature of the media used to cover the label side of the disk. Because of the low signal-to-noise ratio, conventional use of a FES signal configured in a closed-loop feedback circuit will not effectively provide signals to the actuator focus coil which result in convergence on the focal point.
A second difficulty in using the FES signal in a conventional manner is that the OPU is biased at rest to focus at a distance further than the surface of the label side of the disk. This is because the OPU is designed to focus on data pits defined approximately 1.2 mm from the surface of the data side of the disk. Thus, a DC offset must be found to result in movement of the actuator away from the disk surface when focusing on the label side of the disk.
Additionally, tilting of the disk within the optical disk drive and variances in the thickness of the disk produce focus errors that tend to appear as a sinusoidal variation once per revolution of the disk. Similarly, warping of the disk creates focus errors that may appear as a sinusoidal variation twice per revolution. Without an effective closed-loop feedback circuit, these sources of focus error can result in much degraded performance when marking an image to the label side of a disk.
As a result, new and improved systems and methods of focusing the OPU on the label side are needed.